I had this issue on Windows Vista Home Premium and the problem was solved by editing the path environment variable to have these paths at the beginning of the string:

C:\php5\;C:\php5\ext\;

Replace the path with the one that suits your PHP install location.

Even though I had the php.ini set up perfectly, the Windows path environment variable had to be set and the machine re-booted before the PHP extension DLLs (such as for curl or mysql) could be loaded.

And, to get Apache 2.2 to recognize the php.ini in your PHP directory, you need to add:

PHPIniDir "c:/php5"

to the hpptd.conf file.

There is quite a lot of help online regarding configuring the php.ini file but I hope this adds to the mix.